The present invention generally relates to calibration of a medical diagnostic imaging system. In particular, the present invention relates to reconstruction calibration of detector position and source motion based on a multi-pin phantom.
Medical diagnostic imaging systems encompass a variety of imaging modalities, such as x-ray systems, computerized tomography (CT) systems, ultrasound systems, electron beam tomography (EBT) systems, magnetic resonance (MR) systems, and the like. Medical diagnostic imaging systems generate images of an object, such as a patient, for example, through exposure to an energy source, such as x-rays passing through a patient, for example. The generated images may be used for many purposes. For instance, internal defects in an object may be detected. Additionally, changes in internal structure or alignment may be determined. Fluid flow within an object may also be represented. Furthermore, the image may show the presence or absence of items in an object. The information gained from medical diagnostic imaging has applications in many fields, including medicine and manufacturing.
In order to help ensure that medical diagnostic images are reliable, it is advantageous to calibrate medical diagnostic imaging systems. The calibration of imaging systems is important for several reasons, including image quality and system performance. Poor image quality may prevent reliable analysis of an image. For example, a decrease in image contrast quality may yield an image that is not usable clinically. The calibration of medical imaging systems may help to produce a distinct and usable representation of an object.
The calibration of medical diagnostic systems is also important for safety reasons. For example, exposure to excessively high levels of x-ray energy may create certain health risk. Because of the health risk, governmental standards have been established for the use of x-ray systems. The level of x-ray energy emitted by an x-ray system may be measured in terms of radiation dosage. Calibration of x-ray systems and other medical diagnostic imaging systems may ensure that the radiation dosage to which the target is exposed does not exceed clinical standards.
One device that may be used in the calibration of medical diagnostic imaging system parameters, such as image quality and radiation dosage, is called a phantom. Many types of phantoms have been proposed. For example, phantoms may be physical replicas of imaging targets, such as human body parts. Another example of a phantom type is a physics-based phantom. A physics-based phantom may be comprised of various structures affixed to a common base. The structures of a physics-based phantom may possess varying characteristics, such as shape, size, density, composition, and arrangement, for example. Furthermore, physics-based phantoms may be constructed from various materials, including metal and plastic.
The structures of physics-based phantoms may affect characteristics of energy sources, such as x-rays, for example, which pass through the physics-based phantom. For example, metal structures may block x-rays. Additionally, plastic structures may merely decrease the energy level of received x-rays. A pattern resulting from the changes in the energy levels of received x-rays is represented in an x-ray image. The resulting pattern in the x-ray image may be easy to detect and analyze due to factors such as the contrast produced by the difference in received x-ray energy levels.
Phantoms may serve a variety of purposes. For example, phantoms may be used to practice positioning of an imaging target. Additionally, phantoms may be used to test parameters of the medical imaging system. Also, phantoms may be used to gauge the radiation dosage of energy emitted by the medical diagnostic imaging system. Furthermore, phantoms may be used for calibration and image quality assessment. However, for accurate positioning and system calibration, conventional phantoms are expensive and require high precision during manufacture. Thus, there is a need for a phantom that may accurately and easily determine component positions in a medical diagnostic imaging system. There is a need for an inexpensive phantom that may be used to calibrate a medical diagnostic imaging system which does not require high precision during manufacture or use.
In CT imaging systems, for example, an object such as a patient or a phantom is illuminated with x-rays from a plurality of angles to produce a set of x-ray projections. Each of the plurality of detectors in the imaging system samples the x-ray signal a plurality of times, and when the aggregate data from each detector is assembled with sample number on one axis and detector number on the other, the result is referred to as a sinogram. For example, if there are 1728 detectors in a CT system and each detector is sampled 864 times, the sinogram would be a matrix of 864×1728 x-ray attenuation values. The term “sinogram” derives from the sinusoidal shadow a solid object like a pin presents. The CT imaging system calculates or “reconstructs” a two dimensional image data from the sinogram data.
Inaccuracies in the CT imaging system may result in blurring, streaking, or introduction of ghost images or artifacts in the resulting image. For example, if a detector position or the center of a medical imaging system is inaccurate, an x-ray will be projected at an incorrect angle and produce an error in the resulting image. Thus, a need exists for a method and apparatus for more accurate calibration of a medical diagnostic imaging system.
Current calibration methods often involve time intensive or complicated procedures. Frequent calibration is required to help ensure consistent image quality. Additionally, existing calibration methods rely on the assumption that system components, such as detectors, have been accurately positioned and located. That is, conventional systems rely on the manufacturer's stated position of detectors and energy beam source in relation to the center of the imaging system. Accuracy may be time consuming and difficult to achieve, and error in the manufacturer's positioning may result in streaks on the images. Furthermore, current calibration methods require precise positioning of the phantom in order to properly calibrate the imaging system. Thus, a need exists for a method and apparatus for quick and easy system calibration. A need further exists for imaging system calibration using a low-precision phantom.
Additionally, EBT systems utilize a high energy beam of electrons to strike a target and produce x-rays for irradiating an object to be imaged. The point where the electrons strike the target is called the “beam spot.” Dipole, quadrupole, and focusing coils may be used to deflect the electrons along the target to produce x-rays. Motion of the electron beam must be “tuned” to optimize beam motion and more accurately produce a beam spot.
Current methods of tuning EBT scanners involve sweeping the electron beam over “w” shaped wires (“W-wires”) and evaluating the beam spot shape and position as a function of time. W-wires are expensive, however. Thus, there is a need for an inexpensive method of “tuning” or calibrating an electron beam. Additionally, in current EBT systems, only a small number of W-wires may fit (for example, 15 wires in current scanners), reducing accuracy of a tuning correction. Thus, there is a need for a system for more accurately tuning electron beam motion. Furthermore, in current systems, the W-wires are separated from the scanning targets. Therefore, a theoretical transfer function is currently necessary to move the beam from the W-wire target to the scanning target. Thus, a need exists for a method of measuring tune accurately on the scanning target itself, rather than on W-wires. There is a need for direct measurement and modification of electron beam currents based on actual imaging x-rays.